Optical amplifier for CATV system with forward and reverse paths

ABSTRACT

A system for amplifying separate optical signals traveling in a forward path and a reverse path over an optical communications system used for CATV program distribution. A first and second optical circulator, each having a first, second, and third port, are connected to respective opposite sides of a bi-directional amplifier. The first port of the first circulator inputs a first optical signal traveling in the forward path. The second port of the first circulator is coupled to an input/output port of the amplifier. The third port of the first circulator outputs an amplified second signal traveling in the reverse path. The second circulator is similarly connected to the side of the amplifier. A first and second signal/signal WDM coupler, each having a pass port P, an add port A/D, and a common port C, are coupled to the first and second optical circulators, respectively. The first port of the first optical circulator is connected to the pass port of the first signal/signal WDM coupler, and the third port of the first optical circulator is connected to the add port of the first signal/signal WDM coupler. The common port of the first signal/signal WDM coupler outputs an amplified second path signal and inputs a first path signal. The second signal/signal WDM coupler is similarly connected to the second optical circulator and outputs an amplified first path signal and inputs a second path signal. In this manner, the present invention provides bi-directional amplification of signals traveling in opposite paths on a single fiber communications link.

FIELD OF THE INVENTION

This invention relates generally to the amplification of opticalsignals, and is more particularly directed to a system for thebi-directional amplification of linearly modulated optical signals of aCATV system with forward and reverse paths.

BACKGROUND OF THE INVENTION

Conventional subscriber systems, such as CATV systems, are typicallyarranged to provide a variety of services including video, data, audio,and telephony. The services available to the subscribers of a particularsystem are most often provided from a national network source andtransmitted to the headend of the system over a satellite link. Anintegrated receiver-detector (IRD) located at the headend downlinks theservices and additional headend equipment combines the satelliteservices with any local or terrestrial services into a broadband signalfor transportation over the CATV distribution network.

There are many network architectures used in CATV systems for thedelivery of the broadband signal to the subscribers. In the past, acommon network architecture was a tree and branch structure having oneor more coaxial cable trunks which covered the subscriber area. Fromeach trunk, numerous coaxial cable feeders were then run to subscribersin smaller localities. Because optical fiber cable can carry more CATVservices with less loss over greater distances than coaxial cable, manyCATV systems are upgrading their coaxial cable plant with fiber opticcable. One popular modern architecture of a CATV system which usesoptical fiber cable is termed a "star" configuration because it utilizesmany optical fibers carrying the broadband signal from a central pointto a plurality of local service areas or nodes. At each node, thebroadband signal is converted back to a radio frequency (RF) broadbandsignal and distributed to subscribers via a series of coaxial cablefeeders.

Generally, these CATV systems have been deployed as single point tomultipoint broadcast type systems. Present-day CATV systems typicallyuse a forward or downstream path over which the broadband video signalis broadcast, and a reverse or upstream path over which information maybe returned to the headend. The upstream path is generally used forinteractive data from the subscribers or for network control, billing,maintenance, and authorization functions. Interactive data can includedata concerning a subscriber's instructions for ordering a product,feedback about on-screen information such as for interactive gaming orpolling, a subscriber telephony or data signal directed to a personinside or outside the network, or the like. Thus the downstreambroadband signal and the upstream signal for a CATV system aredistinctly different from each other.

In the star architecture using fiber optic cable as the transmissionmedium to a series of nodes, one fiber can be utilized for the forwardpath and another fiber can be utilized for the reverse path. CurrentCATV architectures employ 1550-nm or 1310-nm wavelength optical signalbeams to transport broadband video signals downstream using the forwardpath fiber. Interactive data and telephony traffic is transportedupstream using 1310-nm wavelength optical signal beams over the reversepath fiber. Commercial high power optical amplifiers have made itpossible to transport broadband video signals through forward pathoptical fibers which are deployed deeper, close to the subscriber'spremises using 1550-nm wavelength lasers. Due to longer forward pathfiber runs, reverse path optical fiber distances have proportionallyincreased. Therefore, upstream transmission of data, telephony, andvideo signals using 1310-nm wavelength lasers can be adversely affectedbecause of additional attenuation over the longer reverse path fiber. Analternative for transporting upstream traffic is the use of 1550-nmwavelength laser sources where optical amplifiers can be employed tocompensate for the excess loss in the reverse path fiber.

The use of separate downstream and upstream fibers also necessitates theuse of additional optical amplifiers, which adds system expense. Moresystems are beginning to eliminate the necessity for a separate upstreamfiber cable from each node by using optical multiplexing techniques tocombine the downstream signal and upstream signal on a single fiber.However, because most optical amplifiers are unidirectional, there isstill the additional expense of dual active components for the system.

Therefore, there is a need in the art for a bi-directional opticalamplifier for CATV systems that can accommodate both forward and reversepath traffic. There is also a need for a bi-directional opticalamplifier capable of transmitting, receiving, and regenerating in boththe forward and reverse paths for a star or ring architecture having adual fiber optical cable configuration or for a single fiber system.Further, it would be of advantage if such a bi-directional opticalamplifier could take into account the different characteristics andparameters between the forward path signal and the reverse path signal.

SUMMARY OF THE INVENTION

The present invention provides a system for amplifying separate opticalsignals traveling in a forward path and a reverse path over an opticalcommunications system used for CATV program distribution. Advantageousimplementations are provided for a headend interface amplifier, aregeneration amplifier, and a node interface amplifier. In addition,bi-directional optical amplifiers are provided for single or dualoptical fiber systems.

In accordance with an embodiment of the present invention, a first andsecond optical circulator, each having a first, second, and third port,are connected to respective opposite sides of a bi-directionalamplifier. The first port of the first circulator inputs a first set ofoptical signal beams traveling in the forward path. The second port ofthe first circulator is coupled to an input/output port of theamplifier. The third port of the first circulator outputs an amplifiedsecond set of optical signal beams traveling in the reverse path.Similarly, the first port of the second circulator inputs the secondoptical signal traveling in the reverse path. The second port of thesecond circulator is coupled to the other input/output port of theamplifier. The third port of the second circulator outputs the amplifiedfirst set of optical signal beams traveling in the forward path. Thus,the present invention provides amplification of forward and reverse pathsignals in a dual fiber CATV communications system.

Another embodiment of the present invention includes two signal/signalWDM couplers each having a pass port P, an add (or drop) port A/D, and acommon port C. The first and second signal/signal WDM couplers arecoupled to the first and second optical circulators, respectively. Thefirst port of the first optical circulator is connected to the pass portof the first signal/signal WDM coupler, and the third port of the firstoptical circulator is connected to the add port of the firstsignal/signal WDM coupler. The common port of the first signal/signalWDM coupler outputs an amplified second path signal and inputs a firstpath signal. Similarly, the first port of the second optical circulatoris connected to the pass port of the second signal/signal WDM coupler,and the third port of the second optical circulator is connected to theadd port of the second signal/signal WDM coupler. The common port of thesecond signal/signal WDM coupler outputs an amplified first path signalsand inputs a second path signals. In this manner, the present inventionprovides bi-directional amplification of signals traveling in oppositepaths on a single fiber communications link.

The present invention also provides a method for the transmission ofbroadcast and narrowcast services in a system having a headend, a hubconnected the headend, and a plurality of nodes connected to the hub.Instead of transmitting the signals directly from the headend to eachnode, the signals are transmitted from the headend to the hub. Thesignals transmitted from the headend to the hub include both broadcastservices to be transmitted to each node and narrowcast services to betransmitted only to predetermined targeted nodes. The broadcast andnarrowcast services are transmitted by optical signal beams operating atdifferent wavelengths. Subscribers can transmit signals to the headend,via each node, on the reverse channel. The reverse channel informationfrom subscribers is combined at each respective node into a singlewavelength for each node. The present invention adds the narrowcastservices at the headend instead of at the hub. By adding the narrowcastservices at the headend, the processing of the narrowcast signals isperformed by the electronic processing equipment at the headend.Therefore, the narrowcast signals leave the headend as optical signals,and the hub is only required to perform optical signal processing, andis not required to have any electrical processing equipment. In thismanner, the hub performs only optical signal processing, which reducesthe complexity and cost of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a system schematic diagram of a typical hybrid fiber/coaxial(HF/C) communications system for a conventional CATV system.

FIG. 1B is a system schematic diagram of a WDM HF/C communicationssystem in accordance with the present invention.

FIG. 2 is a pictorial representation of the optical wavelengthallocation and RF signal configuration in the forward and reverse pathoptical beams of the HF/C communications system illustrated in FIG. 1B.

FIG. 3 is a schematic block diagram of an optical communications linkusing a dual fiber transmission architecture between the headend and anode of the system illustrated in FIG. 1B.

FIG. 4 is a schematic block diagram of an optical communication linkusing a single fiber transmission architecture between the headend and anode of the system illustrated in FIG. 1B.

FIG. 5 is a schematic block diagram of two optical amplifiers which areutilized in a simple form for a headend interface amplifier, aregeneration amplifier, and a nodal interface amplifier in the opticalcommunication link illustrated in FIG. 3.

FIG. 6 is a schematic block diagram of a first implementation of anoptical amplifier constructed in accordance with the present inventionwhich is utilized for a headend interface amplifier, a regenerationamplifier, and a nodal interface amplifier in the optical communicationlink illustrated in FIG. 3.

FIG. 7 is schematic block diagram of a second implementation of anoptical amplifier constructed in accordance with the present inventionwhich is utilized for a headend interface amplifier in the opticalcommunication link illustrated in FIG. 4.

FIG. 8 is schematic block diagram of a third implementation of anoptical amplifier constructed in accordance with the present inventionwhich is utilized as a node interface amplifier in the opticalcommunication link illustrated in FIG. 4.

FIG. 9 is a schematic block diagram of a fourth implementation of anoptical amplifier constructed in accordance with the present inventionwhich is utilized as a regenerator amplifier in the opticalcommunications link illustrated in FIG. 4.

FIG. 10 is a detailed schematic diagram of the basic bi-directionaloptical amplifier.

FIG. 11 is a detailed schematic diagram of the optical amplifierillustrated in FIG. 6.

FIG. 12 is a detailed schematic diagram of the optical amplifierillustrated in FIG. 7.

FIG. 13 is a detailed schematic diagram of the optical amplifierillustrated in FIG. 8.

FIG. 14A is a detailed schematic diagram of a first implementation ofthe optical amplifier illustrated in FIG. 9.

FIG. 14B is a detailed schematic diagram of a second implementation ofthe optical amplifier illustrated in FIG. 9.

FIG. 15 is a diagram of an wavelength division multiplexing (WDM)coupler utilized in the optical amplifiers illustrated in FIGS. 11-14B.

FIG. 16 is a diagram of an optical circulator utilized in the opticalamplifiers illustrated in FIGS. 11-14B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now in detail to the drawings, FIG. 1A shows the networkoperating elements of a typical subscriber communications or CATV system10 having a star type hybrid fiber/coaxial (HF/C) architecture. Aheadend 12 is the originating point for a broadband information signalwhich is broadcast to a multiplicity of subscribers 36. Signal sourcesinput to the headend 12 include over-the-air stations, satelliteservices, and terrestrial services, such as by microwave relay. Inaddition, local programming services may also originate at the headend12. These services may include analog video, digital video, voice(telephony), data services, or the like. All of the source signals areprocessed and then combined at the headend 12 into an RF broadbandsignal for broadcast or transmission over a distribution network.

In an HF/C system, the RF broadband information signal is converted intolight by a modulation process before being broadcast. The modulation isan amplitude modulation onto a light source, usually a laser source, fortransmission on optical communications links 16 comprising optical fibercable. The headend 12 feeds the optical broadband information signalthrough the plurality of optical communications links 16 to a pluralityof nodes 14. In a typical system, each fiber communications link 16feeds an associated optical node 14 with the optical broadbandinformation signal. The nodes 14 are arranged in a star architecture toprovide coverage of convenient geographic areas.

The optical broadband information signal is detected by a conventionaloptical receiver at each node 14 and converted back into an RF broadbandinformation signal to be provided to a plurality of coaxial feedercables 22 emanating from the node 14. Each feeder cable 22 terminates attermination block 30 and includes a number of broadband signal taps 24and a lesser number of bi-directional line extender amplifiers 26 and28. The taps 24 and amplifiers 26 and 28 are serially interposed alongsections of the feeder cable 22 between the node 14 and a terminationblock 30. Each broadband signal tap 24 draws a small portion of thebroadband information signal transmitted through feeder cable 22 anddelivers it to a subscriber 36, such as a home or business. Typically,there are hundreds and sometimes thousands of subscribers supported byeach distribution node 14. The broadband signal taps 24 are connected tothe subscriber equipment, such as a network interface unit (NIU) 34,using a subscriber drop cable 32.

Subscribers are able to transmit a return information signal back towardthe headend 12 over a reverse channel by using an RF modulator.Generally, a narrow band, digitally modulated, asynchronous signal istransmitted from each subscriber via the drop cable 32 and feeder cable22 toward the node 14. The reverse path line extender amplifiers 26amplify these signals in the upstream direction where they are collectedfor all the subscribers transmitting on a particular feeder 22. Allreturn information signals from each feeder 22 for a node 14 arecombined at the node and converted into an optical return informationsignal for transmission over the optical communications link 16 to theheadend 12.

FIG. 2 pictorially represents the bandwidth allocation and generalconfiguration of the forward and reverse signals for the communicationsystems illustrated in FIGS. 1A and 1B. The downstream or forward signalgenerally receives the largest portion of system bandwidth allocationbecause it carries the information services to the subscribers 36. Inmany typical CATV systems, the downstream channel includes a bandwidthfrom 50 MHz-550 MHz in older systems and from 50 MHz-750 MHz in newersystems. There is considerable pressure on the downstream channel tocarry even more information and recent CATV systems have expanded theforward channel capacity from 50 MHz-1000 MHz. The downstream broadbandinformation signal is typically a frequency division multiplex of 6 MHzchannels. One major component of the multiplex is video programmingchannels having baseband NTSC formatted video programs amplitudemodulated onto a carrier. Other services can also be provided includingdata and telephony with the analog or digital modulation formats. Thedata, telephony and digital video channels are placed from 550 MHz-1000MHz. These services are known as targeted or "narrowcast" services.

The upstream or reverse channel comprises a broadband signal which is afrequency division multiplex of a plurality of carriers. The reversechannels including AM modulated (QPR, QPSK, nQAM, etc.) channels are ofdiffering bandwidths. Digitally modulated channels are presently beingused for a variety of digital services including broadband telephony anddata services including one way and interactive services. The upstreamchannel typically comprises a frequency division multiplex of carriersmodulated with the channels in the frequency range of 5 MHz-50 MHz.These upstream channels are generally of lesser bandwidth than thedownstream channels, on the order of 50-100 kHz, and many times aredigitally modulated and have a different modulation depth and index.

The upstream channels are generally of lesser bandwidth because theyreturn information from individual subscribers and thus carry lowerinformation loads than the downstream channels and may not be used allof the time. Typically the upstream channels are used to communicate anindividual subscriber's telephony signal, and requests for data, orservices back to the headend. Therefore, the optical communicationslinks 16 are bi-directional communications paths that provide for thetransmission of an optical broadband information signal from the headend12 to the node 14 in a forward path and for the transmission of anoptical return information signal from the node 14 to the headend in areverse path.

Two basic types of services channels are provided by the communicationssystem. One channel is a "broadcast" services channel, which consists ofthose services that are transmitted to all nodes. The other type ofservices channel is a "narrowcast" services channel, which is provided,or targeted, only to a particular node or nodes, but is not necessarilyprovided to all nodes.

As will be understood by those skilled in the art, each node servesdifferent subscribers. Typically, each node serves a differentgeographic region within the communications system. The different areasserved by the different nodes may include subscribers having differentdemographic characteristics. The different regions served by thedifferent nodes, therefore, may include subscribers that desiredifferent communications services. For example, subscribers in a regionhaving a large Hispanic population may wish to be provided withprogramming in the Spanish language. Thus, the communications system mayprovide Spanish-language programming as a narrowcast service channelsuch that the Spanish programming is provided to subscribers in thatparticular node but is not provided to subscribers in another node thatserves a much smaller Hispanic population. Therefore, each node may haveits own set of narrowcast services channels.

FIG. 1B shows a communications system used for the transmission ofbroadcast and narrowcast services channels in accordance with thepresent invention. The system of FIG. 1B includes a headend 12 and a hub13. The hub feeds optical information signals through a plurality ofcommunications links 16 to a plurality of nodes 14. In FIG. 1B, theplurality of nodes have been identified as node 1, node 2, node 3, node4, . . . , node M.

The headend transmits all of the signals to be transmitted to the nodes.Instead of transmitting the signals directly from the headend to eachnode, the signals are transmitted from the headend to the hub. Thesignals transmitted from the headend to the hub include both thebroadcast services channels and the narrowcast services channels. Thebroadcast and narrowcast services channels are transmitted by opticalsignal beams operating at wavelengths chosen from the forward set ofwavelengths λ₁₀, λ₁₁, λ₁₂, λ₁₃, λ₁₄, . . . , λ_(1M), as shown in FIG. 2.The broadcast services channel is transmitted by an optical signal beamoperating, for example, at wavelength λ₁₀. The broadcast channelwavelength is distributed from the hub to all M nodes. The remainingoptical signal beams operating at wavelengths λ₁₁, λ₁₂, λ₁₃, λ₁₄, . . ., λ_(1M) are narrowcast services channels targeted at nodes 1, 2, 3, 4,. . . , M, respectively. Individual targeted optical signal beams atdifferent wavelengths may carry the same or different programming. Thusthe narrowcast services channels, identified as wavelengths λ₁₁, λ₁₂,λ₁₃, λ₁₄, . . . , λ_(1M), are transmitted on the forward channel fromthe hub to node 1, node 2, node 3, node 4, . . . , node M, respectively.It will be understood that the broadcast signal λ₁₀ is transmitted onthe forward channel from the headend to the hub, and then distributedfrom the hub to each node. It will also be understood that allnarrowcast services channels are transmitted on the forward channel fromthe headend to the hub, but that the hub processes such narrowcastchannels and transmits only those narrowcast channels designated foreach node. Subscribers can transmit signals to the headend, via eachnode, on the reverse channel. Preferably, the broadcast and narrowcastsignals are carried by their respective wavelengths to their respectivenodes by a single fiber.

These reverse channel signals are designated as λ₂₁, λ₂₂, λ₂₃, λ₂₄, . .. , λ_(2M), for each respective node. The reverse channel informationfrom subscribers is combined at each respective node into a singlewavelength for each node, such as λ₂₁ for node 1, λ₂₂ for node 2, and soforth for each node. As shown in FIG. 2, each wavelength in the forwardwaveband are optical carriers for the forward broadband signal and eachwavelength in the reverse waveband are optical carriers for the reversesignal. The broadcast wavelength λ₁₀ consists of broadcast serviceschannels having frequencies preferably, for example, from 50 MHz to 550MHz. The narrowcast wavelengths λ₁₁, λ₁₂, λ₁₃, λ₁₄, . . . , λ_(1M),consist of narrowcast services channels having frequencies preferablyfrom 550 MHz to 1000 MHz. Preferably, the reverse channel signals arecarried from each node to the hub for processing on the samebroadcast/narrowcast fiber or on a dedicated second fiber.

In prior systems, the narrowcast services were inserted at the hub.Adding the narrowcast services channels at the hub required that certainelectronic equipment be provided at the hub for processing the RFnarrowcast services and converting them from electrical signals tooptical signals for transmission over optical fiber to the nodes. Also,prior implementations that added narrowcast signals at the hub ratherthan the headend transmitted the narrowcast services to each node over aseparate optical fiber, thereby increasing the system's complexity andcost.

The present invention adds the narrowcast services channels at theheadend instead of at the hub. By adding the narrowcast serviceschannels at the headend, the processing of the narrowcast signals isperformed by the electronic processing equipment at the headend.Therefore, the narrowcast signals leave the headend as optical signals,and the hub is not required to have any electrical processing equipment.In this manner, the hub is not required to have equipment for processingelectrical signals, but is only required to have optical processingequipment for processing the optical signals to and from the headend,which reduces the complexity and cost of the system.

FIG. 3 illustrates a block diagram of a dual fiber architecture for anoptical communication link 16. At the headend 12 of the opticalcommunication link 16 is an optical transmitter 50 which generates anoptical signal modulated with the broadband information signal. Themodulation can be intensity or phase modulation and the source laser canradiate at a number of different wavelengths. Preferably the opticaltransmitter externally phase modulates a 1550-nm laser source with thebroadband information signal. The resulting optical information signalis optically coupled to the input of the forward stage of an opticalheadend interface amplifier 58. The signal is optically amplified andpropagated in the forward direction over a separate optical fiber link64. The optical information signal is then regenerated by the forwardstage of an optical regenerator amplifier 60 before being propagated inthe forward direction to an optical node interface amplifier 62 overanother part of the fiber link 64. The output of the forward stage ofthe node interface amplifier 62 is detected and demodulated by anoptical receiver 54 at node 14 and converted back into an RF informationsignal for distribution over the coaxial cable part of the network.

Similarly, an optical transmitter 56 at node 14 converts the returninformation signal into a modulated optical return signal. Thetransmitter 56 can be the same or different than transmitter 50. Thetransmitters 50 and 56 can transmit on the same frequency or differentwavelengths, usually between 1530-1570-nm. This signal is opticallyamplified by, and transmitted through, the return stages of the nodeinterface amplifier 62, the regenerator amplifier 60, and the headendinterface amplifier 58 via separate optical fiber links 66.

FIG. 4 illustrates an optical link 16 where the dual fiber links havebeen replaced by single optical fibers 74 and 76. The fibers 74 and 76carry both the forward and reverse optical signals between a headendinterface amplifier 68 and a node interface amplifier 72. They areregenerated by one or more regeneration optical amplifiers 70. Theheadend interface amplifier 68 splits the two signals on its input sideand combines the two signals on its output side. Conversely, nodalinterface amplifier 72 combines the two signals on its input side andsplits the two signals on its output side. Thus, both amplifier 68 andamplifier 72 are three port devices. Amplifier 70 combines the twosignals on both sides and is, therefore, a two port device.

Prior implementations of optical communications link 16 utilized a setof amplifiers in the forward direction and a separate set of amplifiersin the reverse direction, each feeding separate optical fiber links 64and 66, as shown in FIG. 3. FIG. 5 is an example showing how eachamplifier 58, 60, and 62 includes two amplifiers to feed the dual fiberlink shown in FIG. 3. One amplifier 63 is used to amplify signals in theforward path and another amplifier 65 is used to amplify signals in thereverse path. However, using dual amplifiers, as shown in the example ofFIG. 5, is relatively expensive.

Optical amplifiers must have output isolators that provide isolation ofthe input for any noise from the optical transport fiber such asRayleigh backscatter (RB), interferometric noise from optical connectorsand fiber splice generated discrete reflections, and stimulatedBrillouin scattering (SBS) noise. In addition, optical amplifiers maypossibly require input isolators to provide isolation of the amplifiedspontaneous emission (ASE) noise and Rayleigh backscatter from theerbium fiber. However, placing optical isolators at the output and inputof a basic optical amplifier will significantly attenuate optical signalbeams traveling in the opposite direction. Therefore, a wide opticalbandwidth (i.e., waveband) optical divider can be implemented at eachend of the optical amplifier. The two oppositely propagating opticalsignal beams are divided into separate optical paths by an opticaldivider which has a common port and two signal ports where a low lossoptical path is provided for one optical signal beam in one directionand a low loss optical path is provided for the other optical signalbeam in the other direction while isolating the two signal ports fromone another. This is an important feature for CATV systems where theforward path signal and the reverse path signal have differentwavelengths, information bandwidth, signal power, modulation format, andtechnique. This allows an independent and transparent amplification ofoptical signal beams at different wavelengths, intensity levels andmodulation depths within a single device.

The present invention provides a system for amplifying optical signalsincluding a bi-directional optical amplifier with a single, dual,triple, or higher optical pump beams. The bi-directional opticalamplifier can have one, two, or more amplification stages and providessuitable signal conditioning and amplification to both the forward pathand reverse path optical signal beams.

In accordance with one embodiment of the present invention, abi-directional four port optical amplifier, such as amplifiers 58, 60,and 62 of FIG. 3, is shown in FIG. 6. A first optical signal beam havinga wavelength λ₁ is input to a forward direction signal port 80,amplified by amplifier 100, and output from a forward direction outputport 82 as the amplified signal beam λ_(1A). Similarly, a second opticalsignal beam having a wavelength λ₂ is input to a reverse directionsignal port 84, amplified by amplifier 100, and output from a reversedirection output port 86 as the amplified signal λ_(2A). Opticalcirculators 110 and 112, as will be described more fully below, routethe first and second signals between the respective ports and theamplifier 100. A single, dual, triple or higher optical pump beams withone, two, or more optical erbium fiber amplification stages can be usedto provide the amplification, although the forward and reverse signalscan be quite different as will be explained more fully hereinafter. Itshould be understood that the signal identified as λ₁, as used herein,refers to the set of forward path optical signal beams λ₁₀, λ₁₁, λ₁₂,λ.sub.₁₃, λ₁₄, . . . , λ_(1M). It should also be understood that signalidentified as λ₂, as used herein, refers to the set of reverse pathoptical signal beams λ.sub.₂₁, λ₂₂, λ₂₃, λ₂₄, . . . , λ_(2M).

Optical circulators 110 and 112 provide separate signal ports for eachoptical signal by coupling their respective second ports to therespective pass ports of the pump/signal WDM couplers 102 and 104 ofamplifier 100. Each optical circulator provides a low loss input opticalpath from the first port to the second port and a low loss opticaloutput path from the second port to the third port. This implementationprovides a four port optical amplifier where a first optical signalpropagating in one direction can be input to the first port of the firstcirculator, amplified, and output from the third port of the secondcirculator while a second optical signal propagating in the oppositedirection can be input to the first port of the second circulator,amplified, and output from the third port of the first circulator.

In accordance with another embodiment of the present invention, abi-directional three port optical amplifier, useful in implementingamplifiers 68 and 72 of FIG. 4, is shown in FIGS. 7 and 8, respectively.The four port amplifier of FIG. 6 can be changed into the three portdevice of headend amplifier 68 and node amplifier 72 by the addition ofa bi-directional combiner on the respective side.

As shown in FIG. 7 with reference to headend amplifier 68, the opticalcombiner 88 is on the output side of amplifier 100. Optical circulator112 routes optical signals between signal ports 80 and 86 and theamplifier 100. For headend amplifier 68, an optical signal having awavelength λ₁ is input to a forward direction signal port 80, amplifiedby amplifier 100, and output from an input/output port 85 as theamplified signal λ_(1A). Similarly, an optical signal having awavelength λ₂ is input to the input/output port 85, amplified and outputfrom a reverse direction output port 86 as the amplified signal λ_(2A).

Node amplifier 72, as shown in FIG. 8, includes an optical combiner 90on the input side of amplifier 72. Optical circulator 110 routes opticalsignals between signal ports 82 and 84 and the amplifier 100. For nodeamplifier 72, an optical signal having a wavelength λ₁ is input to aninput/output port 87, amplified by amplifier 100, and output fromforward direction output port 82 as the amplified signal λ_(1A).Similarly, an optical signal having a wavelength λ₂ is input to thereverse direction input port 84, amplified and output from aninput/output 87 as the amplified signal λ_(2A). It should be evidentthat, if the amplification stage gains in either direction areequivalent or they can be independently varied, only one version existsbecause the combined side may be reversed for use in either application.This is an advantage provided by the invention where an independentlyvariable gain and modulation is provided for both forward and reversepath signals using only one physical gain stage.

In accordance with another embodiment of the present invention, abi-directional two port optical amplifier, useful in implementingregeneration amplifier 70 of FIG. 4, is shown in FIG. 9. The four portamplifier of FIG. 5 can be changed into the two port device of amplifier70 by the addition of combiner 88 on one side and combiner 90 on theother side. As shown in FIG. 9, an optical signal having a wavelength λ₁is input to an input/output port 87 of regeneration amplifier 70. Thesignal is amplified by amplifier 100 and output from an input/outputport 85 as amplified signal λ_(1A). Similarly, an optical signal havinga wavelength λ₂ is input to the input/output port 85, amplified byamplifier 100, and output from the input/output port 87 as amplifiedsignal λ_(2A).

FIGS. 10-14B will now be more fully discussed to describe theimplementations of the versions of optical amplifiers disclosed in FIGS.6-9. It will be understood that the optical amplifier 100 shown in FIG.10 is utilized in amplifiers 58, 60, 62, 68, 70, and 72 shown in FIGS.3-9.

As shown in FIG. 10, bi-directional optical amplifier 100 amplifies afirst optical signal beam λ₁ propagating in a first direction throughthe amplifier to provide an amplified optical signal beam λ_(1A) andamplifies a second optical signal λ₂ propagating in an oppositedirection through the amplifier to provide an amplified optical signalλ_(2A). The amplifier 100 includes two pump/signal WDM couplers 102 and104, pump excitation light sources 106 and 108, and a length of opticalfiber 109. The optical fiber 109 includes a rare earth element dopingwhich can be pumped by an external excitation source, i.e., light, toform a population inversion and optical amplification. Preferably, therare earth element is erbium which produces an optical fiber which canbe pumped at several different wavelengths to provide the desiredoptical amplification.

Each pump/signal WDM coupler 102 and 104 is a 980/1550 nm or a 1480/1550nm coupler that has a pass port P, an add port A/D, and a common port C.The optical erbium fiber 109 is optically coupled at one end to thecommon port C of the first pump/signal WDM coupler 102 and at the secondend to the common port C the second pump/signal WDM couplers 104.Respective pump sources 106 and 108 are coupled to respective WDMcouplers 102 and 104 at the add port. The respective pass ports of WDMcouplers 102 and 104, are connected to input/output ports 101 and 103,respectively, of the amplifier. A low loss forward path for each opticalsignal is provided between the pass port and common port of eachrespective pump/signal WDM coupler. Thus, each WDM coupler combines aseparate optical information signal beam λ₁, λ₂ with respective pumpsource energies λ_(P1) and λ_(P2) and launches the combination into theoptical erbium fiber 109 from its respective end. This produces in theoptical erbium fiber 109 a first optical information signal beam λ₁propagating in one direction and a second optical information signalbeam λ₂ propagating in the other direction, both information signalbeams being amplified by respective pump source energies λ_(P1) andλ_(P2). Each pump/signal WDM couplers also provides isolation betweenthe first and second optical signals.

FIG. 11 shows the amplifiers 58, 60, and 62 in greater detail. Inaccordance with the first embodiment of the present invention, opticalcirculators 110 and 112, each having a first, second, and third port,are connected on opposite sides of amplifier 100. The first port ofcirculator 112 inputs the first optical signal traveling in the forwardpath. The second port of circulator 112 is coupled to input/output port101 of amplifier 100. The third port of circulator 112 outputs theamplified second signal traveling in the reverse path. Similarly, thefirst port of circulator 110 inputs the second optical signal travelingin the reverse path. The second port of circulator 110 is coupled toinput/output port 103 of amplifier 100. The third port of circulator 110outputs the amplified first signal traveling in the forward path.

Thus, the present invention as described with reference to FIG. 11 isadvantageous for optical amplifiers of a dual fiber CATV system. Foramplifiers 58, 60, and 62, the forward path fiber is connected to thefirst port of circulator 112 and the amplified forward path signal isoutput through the third port of circulator 110. Similarly, the reversepath fiber is connected to the first port of circulator 110 and theamplified reverse path signal is output through the third port ofcirculator 112.

FIG. 12 shows the headend amplifier 68 in greater detail in accordancewith an embodiment of the present invention. As shown in FIG. 12, asignal/signal WDM coupler 114 having a pass port P, an add port A/D, anda common port C, is coupled to circulator 110. It will be understoodthat circulator 110 and WDM coupler 114 constitute the combiner 88 ofheadend amplifier 68, as shown in FIG. 7. The pass port of WDM coupler114 is coupled to the first port of circulator 110 and the add port ofWDM coupler 114 is coupled to the third port of circulator 110.

FIG. 13 shows the node amplifier 72 in greater detail in accordance withanother embodiment of the present invention. As shown in FIG. 13, asignal/signal WDM coupler 116 having a pass port P, an add port A/D, anda common port C, is coupled to circulator 112. It will be understoodthat circulator 112 and WDM coupler 116 constitute the combiner 90 ofnode amplifier 72, as shown in FIG. 8. The pass port of WDM coupler 116is coupled to the first port of circulator 112 and the add port of WDMcoupler 116 is coupled to the third port of circulator 112.

Thus, the present invention is advantageous for optical amplifiers of asingle fiber CATV system. For a headend amplifier as shown in FIG. 12,one side of the amplifier has combined signals and the other side hasseparate signals. For example, at the headend, the fiber transmitter ofthe forward path is directly coupled to the amplifier by the signalinput of the optical circulator 112 and the optical receiver of thereverse path is directly coupled to the signal output of the opticalcirculator 112. Conversely, on the other side of the amplifier, theports of optical circulator 110 are coupled to WDM coupler 114 so thatthe input signal λ₂ and output signal λ_(1A) can be transmitted on asingle optical fiber. At a fiber node, the equipment is reversed, asshown in FIG. 13. The single optical fiber is coupled to the networkside of the amplifier by WDM coupler 116 and the fiber receiver of thenode for the forward path is directly coupled to the output signal port,i.e., port 3, of the optical circulator 110. Similarly, the opticaltransmitter for the reverse path is coupled directly to the signal inputport, port 1, of the optical circulator 110.

It should be understood that an additional optical circulator can beused instead of each signal/signal WDM coupler, e.g., WDM couplers 114and 116 shown in FIGS. 12 and 13. For example, another circulator havingthree ports can be connected to circulator 110 such that the third portof the additional circulator is connected to the first port ofcirculator 110 and the first port of the additional circulator isconnected to the third port of circulator 110. The second port of theadditional circulator is used to input and output signals from the fiberoptic communications link. The use of an additional circulator in thismanner allows a forward path signal and a reverse path signal to travelin opposite directions on the same fiber optic cable.

FIG. 14A shows one implementation of the regeneration amplifier 70 ingreater detail in accordance with another embodiment of the presentinvention. As shown in FIG. 14A, two signal/signal WDM couplers 114 and116, each having a pass port P, an add port A/D, and a common port C,are coupled to circulators 110 and 112, respectively. It will beunderstood that circulator 110 and WDM coupler 114 constitute combiner88 and circulator 112 and WDM coupler 116 constitute combiner 90 of thenode amplifier 72, as shown in FIG. 9. The ports of the circulators andWDM couplers are connected as described above.

FIG. 14B shows a second implementation of the regeneration amplifier. Asshown in FIG. 14B, optical circulator 118 is coupled to opticalcirculator 110 and circulator 120 is coupled to circulator 112. It willbe understood that circulators 110 and 118 constitute combiner 88 andcirculators 112 and 120 constitute combiner 90 of the node amplifier 72,as shown in FIG. 9. The first port of circulator 120 is connected to thethird port of circulator 112. The third port of circulator 120 isconnected to the first port of circulator 112. The second port ofcirculator 120 inputs a first signal traveling on the forward path andoutputs an amplified second signal traveling on the reverse path.Circulator 118 is coupled to circulator 110 in a similar manner.

It should be understood that the present invention is not limited to theamplification of only one signal in each direction. Additional signalscan be added by a multiplexer either at the node and/or at the headendand can be amplified in accordance with the present invention.Therefore, although the description of the present invention hereindescribes the present invention with reference to a single forward pathsignal and a single reverse path signal, the present invention is notlimited to the transmission of these two optical signals.

The operation of each WDM coupler, as used in the present invention, isdescribed as follows with reference to FIG. 15. Each WDM coupler, forexample, WDM coupler 114, is a filter type WDM coupler that includes alens filter 115 and a GRIN lenses (not shown). The lens filter 115 iscoated with a material that reflects optical signal beams at certainwavelengths and passes optical signals at certain other wavelengths.Optical signals at wavelengths λ₁ and λ₂ can propagate in the same oropposite directions along the fiber connected to the common port of theWDM coupler. The optical signal beam at wavelength λ₁ is reflected atthe interface of the lens filter and focused out of the fiber at thecommon port. The optical signal beam at wavelength λ₂ passes through thelens filter and is focused into the fiber at the pass port.

As discussed above, two optical signals are combined using abi-directional combiner, preferably comprised of a signal/signal WDMcoupler and an optical circulator. The operation of the opticalcirculator is generally described with reference to FIG. 16. Opticalcirculator 110 includes two bulk optic isolators 120 and 122 placed ontop of each other. The bulk isolators pass oppositely propagatingoptical signal beams. Two or more oppositely propagating optical signalbeams λ₁ and λ₂ are focused into a single-mode fiber connected to port 2by a Graded Index (GRIN) lens at port 2 (not shown). Optical signal beamλ₂, which is focused into isolator 120 at port 2, passes through theisolator into the single-mode fiber at port 3. Similarly, optical signalbeam at wavelength λ₁ is focused into isolator 122 at port 1, and isfocused into the single-mode fiber at port 2. The isolation or rejectionof optical beams from port 2 to port 1 and from port 3 to port 2 shouldbe greater than 30 dB to maintain acceptable video performance. Thecirculator 110 has sufficiently wide bandwidth to pass optical signalbeams propagating in opposite directions at several differentwavelengths. The usable optical bandwidth for commercially availablecirculators and isolators is from 1530-1570 nm. The optical beams can bespaced at regular or irregular wavelength intervals.

While the invention has been described in connection with a preferredembodiment, this specification is not intended to limit the scope of theinvention to the particular forms set forth, but, on the contrary, it isintended to cover any such alternatives, modifications, and equivalentsas may be included within the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A system for amplifying optical signals of acommunications system including modulated signals of CATV systems havinga first path and a second path, including a bi-directional opticalamplifier having a first port and a second port, and where said opticalamplifier amplifies a first path signal input to its first port andoutputs the amplified first path signal through its second port, andwhere said optical amplifier amplifies a second path signal input to itssecond port and outputs the amplified second path signal through itsfirst port, said system comprising:a first optical circulator having afirst, second, and third port, said first optical circulator providing alow loss path from its first port to its second port and from its secondport to its third port; wherein the first path signal is input to thefirst port of said first optical circulator and output through thesecond port of said first optical circulator, wherein said second portof said first optical circulator is connected to the first port of saidoptical amplifier, and said third port of said first optical circulatoroutputs an amplified second path signal; a second optical circulatorhaving a first, second, and third port, said second optical circulatorproviding a low loss path from its first port to its second port andfrom its second port to its third port; wherein a second path signal isinput to the first port of said second optical circulator and outputthrough the second port of said second optical circulator, wherein saidsecond port of said second optical circulator is connected to the secondport of said optical amplifier, and said third port of said secondoptical circulator outputs an amplified first path signal; wherein saidoptical amplifier comprises:at least one stage of a rare earth dopedoptical fiber having a first end and a second end; a first pump/signalwavelength division multiplexing (WDM) coupler having a pass port, anadd port, and a common port; a first pump light source for providingoptical amplification of a first path signal traveling through theoptical fiber in a first path; wherein the first end of said opticalfiber is coupled to the common port of said first pump/signal WDMcoupler, and wherein said first pump light source is coupled to the addport of said first pump/signal WDM coupler; wherein said pass port ofsaid first pump/signal WDM coupler comprises the first port of saidamplifier, such that said first pump/signal WDM coupler provides acombination of said first path signal and said first pump light sourceto the first end of said optical fiber; a second pump/signal wavelengthdivision multiplexing (WDM) coupler having a pass port, an add port, anda common port; a second pump light source for providing opticalamplification of a second path signal traveling through the opticalfiber in a second path; wherein the second end of said optical fiber iscoupled to the common port of said second pump/signal WDM coupler, andwherein said second pump light source is coupled to the add port of saidsecond pump/signal WDM coupler; wherein said pass port of second firstpump/signal WDM coupler comprises the second port of said amplifier,such that said second pump/signal WDM coupler provides a combination ofsaid second path signal and said second pump light source to the secondend of said optical fiber.
 2. The system as set forth in claim 1 whereinsaid optical fiber is an erbium doped optical fiber.
 3. A system foramplifying optical signals of a communications system includingmodulated signals of CATV systems having a first path and a second path,including a bi-directional optical amplifier having a first port and asecond port, and where said optical amplifier amplifies a first pathsignal input to its first port and outputs the amplified first pathsignal through its second port, and where said optical amplifieramplifies a second path signal input to its second port and outputs theamplified second path signal through its first port, said systemcomprising:a first optical circulator having a first, second, and thirdport, said first optical circulator providing a low loss path from itsfirst port to its second port and from its second port to its thirdport; wherein the first path signal is input to the first port of saidfirst optical circulator and output through the second port of saidfirst optical circulator, wherein said second port of said first opticalcirculator is connected to the first port of said optical amplifier, andsaid third port of said first optical circulator outputs an amplifiedsecond path signal; a second optical circulator having a first, second,and third port, said second optical circulator providing a low loss pathfrom its first port to its second port and from its second port to itsthird port; wherein a second path signal is input to the first port ofsaid second optical circulator and output through the second port ofsaid second optical circulator, wherein said second port of said secondoptical circulator is connected to the second port of said opticalamplifier, and said third port of said second optical circulator outputsan amplified first path signal; a third optical circulator having afirst, second, and third port, said third optical circulator providing arespective low loss path from its first port to its second port and fromits second port to its third port; wherein the first port of said firstoptical circulator is connected to the third port of said third opticalcirculator, and the third port of said first optical circulator isconnected to the first port of said third optical circulator; andwherein the second port of said third optical circulator inputs thefirst path signal and outputs an amplified second path signal.
 4. Thesystem as set forth in claim 3 further comprising:a first signal/signalwavelength division multiplexing (WDM) coupler having a pass port, anadd port, and a common port; wherein the first port of said secondoptical circulator is connected to the pass port of said firstsignal/signal WDM coupler, and said third port of said second opticalcirculator is connected to the add port of said first signal/signal WDMcoupler; wherein the common port of said first signal/signal WDM coupleroutputs an amplified first path signal and inputs a second path signal.5. The system as set forth in claim 3 further comprising:a fourthoptical circulator having a first, second, and third port, said fourthoptical circulator providing a respective low loss path from its firstport to its second port and from its second port to its third port;wherein the first port of said second optical circulator is connected tothe third port of said fourth optical circulator, and the third port ofsaid second optical circulator is connected to the first port of saidfourth optical circulator; and wherein the second port of said fourthoptical circulator inputs the second path signal and outputs anamplified first path signal.
 6. The system as set forth in claim 5wherein said first path is a forward path and said second path is areverse path, and wherein said first optical signal is modulated by theforward path signal of a CATV system and the second optical signal ismodulated by the reverse path signal of the CATV system.
 7. The systemas set forth in claim 6 wherein:said forward path signal comprises aplurality of FDM RF signals each having a respective channel carrierwithin the frequency range of approximately 50 MHz to 1000 MHz on aplurality of optical carrier beams having different wavelengths; andsaid reverse path signal comprises a plurality of FDM RF signals eachhaving a respective channel carrier within the frequency range ofapproximately 5 MHz to 50 MHz on a plurality of optical carrier beamshaving different wavelengths.
 8. The system as set forth in claim 5wherein said optical amplifier comprises:at least one stage of a rareearth doped optical fiber having a first end and a second end; a firstpump/signal wavelength division multiplexing (WDM) coupler having a passport, an add port, and a common port; a first pump light source forproviding optical amplification of a first path signal traveling throughthe optical fiber in a first path; wherein the first end of said opticalfiber is coupled to the common port of said first pump/signal WDMcoupler, and wherein said first pump light source is coupled to the addport of said first pump/signal WDM coupler; wherein said pass port ofsaid first pump/signal WDM coupler comprises the first port of saidamplifier, such that said first pump/signal WDM coupler provides acombination of said first path signal and said first pump light sourceto the first end of said optical fiber; a second pump/signal wavelengthdivision multiplexing (WDM) coupler having a pass port, an add port, anda common port; a second pump light source for providing opticalamplification of a second path signal traveling through the opticalfiber in a second path; wherein the second end of said optical fiber iscoupled to the common port of said second pump/signal WDM coupler, andwherein said second pump light source is coupled to the add port of saidsecond pump/signal WDM coupler; wherein said pass port of second firstpump/signal WDM coupler comprises the second port of said amplifier,such that said second pump/signal WDM coupler provides a combination ofsaid second path signal and said second pump light source to the secondend of said optical fiber.
 9. The system as set forth in claim 8 whereinsaid optical fiber is an erbium doped optical fiber.
 10. A system foramplifying optical signals of a communications system includingmodulated signals of CATV systems having a first path and a second path,including a bi-directional optical amplifier having a first port and asecond port, and where said optical amplifier amplifies a first pathsignal input to its first port and outputs the amplified first pathsignal through its second port, and where said optical amplifieramplifies a second path signal input to its second port and outputs theamplified second path signal through its first port, said systemcomprising:a first optical circulator having a first, second, and thirdport, said first optical circulator providing a low loss path from itsfirst port to its second port and from its second port to its thirdport; wherein the first path signal is input to the first port of saidfirst optical circulator and output through the second port of saidfirst optical circulator, wherein said second port of said first opticalcirculator is connected to the first port of said optical amplifier, andsaid third port of said first optical circulator outputs an amplifiedsecond path signal; a second optical circulator having a first, second,and third port, said second optical circulator providing a low loss pathfrom its first port to its second port and from its second port to itsthird port; wherein a second path signal is input to the first port ofsaid second optical circulator and output through the second port ofsaid second optical circulator, wherein said second port of said secondoptical circulator is connected to the second port of said opticalamplifier, and said third port of said second optical circulator outputsan amplified first path signal; a first signal/signal wavelengthdivision multiplexing (WDM) coupler having a pass port, an add port, anda common port; wherein the first port of said first optical circulatoris connected to the pass port of said first signal/signal WDM coupler,and the third port of said first optical circulator is connected to theadd port of said first signal/signal WDM coupler; wherein the commonport of said first signal/signal WDM coupler outputs an amplified secondpath signal and inputs a first path signal.
 11. The system as set forthin claim 10 wherein said first path is a forward path and said secondpath is a reverse path, and wherein said first optical signal ismodulated by the forward path signal of a CATV system and the secondoptical signal is modulated by the reverse path signal of the CATVsystem.
 12. The system as set forth in claim 11 wherein:said forwardpath signal comprises a plurality of FDM RF signals each having arespective channel carrier within the frequency range of approximately50 MHz to 1000 MHz on a plurality of optical carrier beams havingdifferent wavelengths; and said reverse path signal comprises aplurality of FDM RF signals each having a respective channel carrierwithin the frequency range of approximately 5 MHz to 50 MHz on aplurality of optical carrier beams having different wavelengths.
 13. Thesystem as set forth in claim 10 wherein said optical amplifiercomprises:at least one stage of a rare earth doped optical fiber havinga first end and a second end; a first pump/signal wavelength divisionmultiplexing (WDM) coupler having a pass port, an add port, and a commonport; a first pump light source for providing optical amplification of afirst path signal traveling through the optical fiber in a first path;wherein the first end of said optical fiber is coupled to the commonport of said first pump/signal WDM coupler, and wherein said first pumplight source is coupled to the add port of said first pump/signal WDMcoupler; wherein said pass port of said first pump/signal WDM couplercomprises the first port of said amplifier, such that said firstpump/signal WDM coupler provides a combination of said first path signaland said first pump light source to the first end of said optical fiber;a second pump/signal wavelength division multiplexing (WDM) couplerhaving a pass port, an add port, and a common port; a second pump lightsource for providing optical amplification of a second path signaltraveling through the optical fiber in a second path; wherein the secondend of said optical fiber is coupled to the common port of said secondpump/signal WDM coupler, and wherein said second pump light source iscoupled to the add port of said second pump/signal WDM coupler; whereinsaid pass port of second first pump/signal WDM coupler comprises thesecond port of said amplifier, such that said second pump/signal WDMcoupler provides a combination of said second path signal and saidsecond pump light source to the second end of said optical fiber. 14.The system as set forth in claim 13 wherein said optical fiber is anerbium doped optical fiber.
 15. The system as set forth in claim 10further comprising:a second signal/signal wavelength divisionmultiplexing (WDM) coupler having a pass port, an add port, and a commonport; wherein the first port of said second optical circulator isconnected to the pass port of said second signal/signal WDM coupler, andsaid third port of said second optical circulator is connected to theadd port of said second signal/signal WDM coupler; wherein the commonport of said second signal/signal WDM coupler outputs an amplified firstpath signal and inputs a second path signal.
 16. The system as set forthin claim 15 wherein said first path is a forward path and said secondpath is a reverse path, and wherein said first optical signal ismodulated by the forward path signal of a CATV system and the secondoptical signal is modulated by the reverse path signal of the CATVsystem.
 17. The system as set forth in claim 16 wherein:said forwardpath signal comprises a plurality of FDM RF signals each having arespective channel carrier within the frequency range of approximately50 MHz to 1000 MHz on a plurality of optical carrier beams havingdifferent wavelengths; and said reverse path signal comprises aplurality of FDM RF signals each having a respective channel carrierwithin the frequency range of approximately 5 MHz to 50 MHz on aplurality of optical carrier beams having different wavelengths.
 18. Thesystem as set forth in claim 15 wherein said optical amplifiercomprises:at least one stage of a rare earth doped optical fiber havinga first end and a second end; a first pump/signal wavelength divisionmultiplexing (WDM) coupler having a pass port, an add port, and a commonport; a first pump light source for providing optical amplification of afirst path signal traveling through the optical fiber in a first path;wherein the first end of said optical fiber is coupled to the commonport of said first pump/signal WDM coupler, and wherein said first pumplight source is coupled to the add port of said first pump/signal WDMcoupler; wherein said pass port of said first pump/signal WDM couplercomprises the first port of said amplifier, such that said firstpump/signal WDM coupler provides a combination of said first path signaland said first pump light source to the first end of said optical fiber;a second pump/signal wavelength division multiplexing (WDM) couplerhaving a pass port, an add port, and a common port; a second pump lightsource for providing optical amplification of a second path signaltraveling through the optical fiber in a second path; wherein the secondend of said optical fiber is coupled to the common port of said secondpump/signal WDM coupler, and wherein said second pump light source iscoupled to the add port of said second pump/signal WDM coupler; whereinsaid pass port of second first pump/signal WDM coupler comprises thesecond port of said amplifier, such that said second pump/signal WDMcoupler provides a combination of said second path signal and saidsecond pump light source to the second end of said optical fiber. 19.The system as set forth in claim 18 wherein said optical fiber is anerbium doped optical fiber.